1. Field of the Invention
This invention relates generally to set retarded cement compositions and methods of cementing across zones in wells, and more particularly, but not by way of limitation, to set retarded cement compositions having enhanced compressive strength development after placement and methods of cementing across zones in wells using such compositions.
2. Description of the Prior Art
In cementing operations carried out in oil, gas and water wells, a hydraulic cement is normally mixed with sufficient water to form a pumpable slurry and the slurry is pumped across a subterranean zone or zones to be cemented by way of the wellbore penetrating such zone. After placement across the zone or zones, the cement slurry sets into a hard mass.
While cement compositions are utilized in carrying out a variety of operations in wells to accomplish a variety of purposes, cement compositions are most commonly used in primary cementing whereby casing and/or liners are bonded within the wellbore to the formations penetrated thereby. Cementing is accomplished by introducing a cement composition into the annular space between the casing or liner and the wellbore, generally by pumping the cement composition downwardly within the casing or liner to the bottom thereof and then upwardly into the annulus, and then allowing the cement composition to set into a hard mass therein.
One or more of the formations or zones adjacent the cemented annulus can contain fluids under pressure which enter and flow through the cement-containing annulus when the cementing procedure carried out therein is faulty or inadequate. The most common problem of this nature is the flow or migration of gas in the cemented annulus. Such gas can flow to the surface, create communication between producing or other subterranean formations or zones and can, when in high enough volume, create blowouts during the cementing operation between when the cement composition is placed and before the cement composition has set into a hard mass. Minor interzonal gas flow problems can lower production. When the magnitude of leakage requires remedial action, expensive secondary cementing procedures must be carried out.
The occurrence of annular gas flow in a cemented casing-wellbore annulus is believed to relate to the inability of the cement slurry to transmit hydrostatic pressure during the transition of the slurry from a true fluid to a hard, set mass. During this transition phase, initial hydration has begun and the slurry starts to develop static gel strength. Although the system has no compressive strength at this point, the cement column becomes partially self-supporting. This is a very critical period as far as potential gas entry into the cement column is concerned. That is, although the original hydrostatic pressure is trapped within the gelled cement matrix, any volume reductions of the aqueous phase at this point result in rapid decreases in the hydrostatic pressure due to the low compressibility of the fluid phase. Such volume reductions usually occur due to the ongoing hydration reactions and due to the loss of part of the fluid phase to the formation (fluid loss). In this situation, it is possible for the pressure within the cement matrix to fall below the gas entry limit and for gas migration to occur.
Static gel strength is the development of some internal rigidity in the matrix of the cement that will resist a force placed upon it. The development of static gel strength will start to occur immediately after pumping has stopped and will continue to increase until the cement is set. At some time before actual set, the cement will develop a static gel strength high enough to prevent a fluid from moving through it. Tests have indicated that a gel strength of 500 pounds per 100 square feet is sufficient to prevent any movement, although at certain conditions such gel strength can be considerably lower. When the cement has developed a static gel strength high enough to prevent formation fluids from moving through it, the cement is said to have completed its transition phase.
Volume reduction in the cement column can occur as a result of fluids lost from the cement slurry to the formation. Even when fluid loss is very low, small amounts of fluid are still lost from the slurry which can result in a pressure drop in the cement column during the transition phase thereof. Additionally, as the cement in the cement slurry hydrates, a volume reduction caused by such hydration results. Hydration volume reduction can ultimately be as high as three percent. Where the static gel strength development is slow and the volume reduction due to hydration and fluid loss are appreciable, the hydrostatic pressure exerted by the cement column on adjacent formations can drop below the pressure of formation fluids thereby allowing the fluids to enter the cement-filled annulus. If the gel strength of the cement slurry is not high enough to prevent further movement of formation fluids, a fingering or migration phenomena will occur and annular gas leakage and/or interzonal communication will ultimately result. However, where the gel strength is high enough, the flow of formation fluids through the cement column is prevented.
Traditionally, the petroleum industry has attempted to prevent annulus formation fluid flow by increasing the cement slurry density, improving mud displacement, controlling mud-cement slurry compatibility, using fluid loss control additives, causing the cement slurry to expand after setting, and multiple stage cementing. Although these techniques are helpful and have shown some measure of success, none have completely solved the problems. New techniques using cement slurries containing gas whereby the cement slurries are compressible have attained a much greater degree of success. Such techniques are the subject matter of U.S. Pat. Nos. 4,304,298 and 4,340,427.
Another problem often encountered in cementing relates to the cement slurry developing compressive strength at a slow rate and/or the compressive strength development of the in-place cement column not being uniform. With the drilling of wells for the production of hydrocarbons to increased depths during recent years, extended cementing times are required to mix cement compositions and pump them into the annular space in the wells. In addition, at the greater depths, elevated temperatures are encountered which accelerate the normal setting rates of cement compositions to the point where the pumping times, i.e., the mixing and placement times, exceed the pumpable times of the cement compositions, making it difficult or impossible to place the cement compositions at the desired locations in the wells. In order to increase the pumpable times of cement compositions, various set retarding additives have been utilized in cement compositions. While such additives successfully extend the pumpable times between mixing and the setting of cement compositions, they are temperature sensitive, i.e., the higher the temperature of the cement slurry, the greater the quantity of set retarder additive required. In cementing operations, especially when a long liner is involved, the static temperature of the cement column at the top thereof after placement can be considerably lower than the static temperature of the column at the bottom of the annulus, i.e., at the bottom hole static temperature (BHST). In some cementing applications, the static temperature of the cement column at the top can be as much as 40.degree. F. cooler than the BHST. In these applications, the cement slurries must contain set retarders in quantities to achieve required pumpable times at the highest temperature to which the cement slurry is heated, and consequently, after placement the cooler slurry at the top of the cement column can take an excessive time to set and to develop compressive strength whereby the continuation of well operations is delayed.
By the present invention, set retarded cement compositions for cementing across zones in wells are provided which have enhanced gel strength and compressive strength development after placement in the annulus. That is, the cement compositions develop high gel strength in a short period of time after placement followed by rapid compressive strength development. The rapid development of high gel strength in a short time span prevents fluid invasion into the annulus containing the cement composition even though the hydrostatic pressure of the cement column may fall below the pressure of formation fluids during the transition of the slurry to a solid mass. Further, the enhanced and uniform compressive strength development of the cement column in spite of a temperature differential over the length of the column shortens the down time required as a result of carrying out the cementing procedure.